In electrical appliance such as personal computer and image forming apparatus, attention has been focused on reducing standby power consumption to a minimum level. One known solution is to incorporate a power supply device that stops power supply from a main power supply circuit during standby time.
However, an electrical appliance on standby is sometimes required to return to a normal operation mode in response to external signals. During standby time, a facsimile, for example, needs to be ready to appropriately receive facsimile data input externally over telephone lines. A printer needs to return to a normal operation state immediately on detection of image data input from a personal computer and perform an image forming operation according to the input image data.
Japanese Patent Application Laid-Open No. 2003-63101 discloses a composite machine including: a power saving controller for controlling power supply from a main power supply means to a main controller (main CPU) and detecting a trigger for cancellation of a power-saving operation mode; and an auxiliary power supply means for supplying power to the power saving controller. The composite machine stops power supply from the main power supply means in the power-saving operation mode, thereby reducing standby power consumption.
The composite machine of Japanese Patent Application Laid-Open No. 2003-63101, however, still consumes power more than necessary in the power-saving operation mode. The composite machine consumes power to be capable of communicating with external devices in the power-saving operation mode. In the composite machine, power is supplied to a sub CPU that performs power saving control and a bunch of interface sections, such as a FAX board and a PC interface board, that communicate with external devices.
Although the composite machine takes some malfunction prevention measures, the composite machine is likely to malfunction when a power-save request and a start-up request are input to the composite machine at about the same time with a slight time lug, or when noise are added to a power-save request to be input to the composite machine.
A feature of the present invention is to offer a power supply device and a communication system that are capable of receiving external signals with minimum power consumption in a power-saving operation mode.
Another feature of the present invention is to offer a power supply device and a communication system that are capable of preventing waste of power caused by false detection of input signal.